Field-effect transistors are one type of electronic component used in the fabrication of integrated circuitry. Such include a pair of source/drain regions have a channel region received therebetween. A gate is received proximate the channel region and separated therefrom by a gate dielectric. By applying suitable voltage to the gate of the transistor, the channel region becomes electrically conductive. Accordingly, the transistor switches from a non-conductive state to a conductive state upon application of a suitable threshold voltage to the gate. It is desirable to keep threshold voltages of transistors small and also to keep power consumption of transistors low. One significant property of the gate which is determinative of threshold voltage is work function. It is the work function of the gate, together with the doping level of the channel region, which determines the threshold voltage of a field-effect transistor device. To keep threshold voltages of transistors small and power consumption low, it is desirable that the work function of the gate material be approximately equal to the work function of the material of the channel region.
Usually, not all transistors of an integrated circuit are of the same construction or materials. Accordingly, it is recognized and often desirable that different transistor gates be fabricated to have at least two different work functions. One manner of providing different work functions is to provide different gate electrodes to be formed of different materials. For example for conductive polysilicon, using different conductivity-enhancing dopants and concentrations may provide different work functions for different transistors. For metal gates, use of different metals, or quantities of metals in metal alloys, are also known to impact work function in the finished device.